Temperatures obtained in conventional high frequency microwave ovens, which impart surface temperatures to foods of approximately 200.degree. F., are insufficient to brown and crisp food products. Cooking temperatures in conventional radiant ranges of 250.degree.-600.degree. F. are required for effective browning and crisping of foods. To simulate radiant heat sources in microwave cooking, the food packaging industry has employed microwave susceptor materials which impart radiant temperature levels to food surfaces in the presence of microwave energy. Such susceptor materials have found wide application in providing disposable ovenable food containers made of paper and paperboard.
Conventional susceptor materials are fabricated by depositing a film of conductive elemental metal on a non-interactive or microwave transparent supporting substrate, which may be a biaxially oriented polyester film, by the process of vacuum vapor deposition or sputtering onto the plastic film. Such metallized substrates are laminated to paper, paperboard or other rigid materials to provide microwave interactive disposable food packaging (see U.S. Pat. No. 4,267,420 to Brastad, 4,641,005 to Seitferth or 4,733,513 to Watkins). Microwave energy interacts with the conductive metal coating to generate heat and provide a susceptor feature. However, conventional metalized films are not entirely satisfactory in that they require use of adhesives in the fabrication of the packaging. Adhesives may emit volatile chemicals at high temperatures generated by the conductive metal coating presenting a health hazard. Thus adhesives require additional processing and safety specifications to meet FDA safety requirements.
It has been proposed that microwave susceptor characteristics may be provided through use of conductive polymers. U.S. Pat. No. 4,892,782 to Fisher et al. discloses microwave packaging materials comprising drapable, liquid permeable, woven or nonwoven fibrous dielectric substrates, coated with susceptor materials including metal alloys or conductive polymers. Microwave packaging materials are provided with a means for removing liquid by-products. During microwave cooking of food items the susceptor heats up and moisture evolves as vapors, hastening the browning and crisping of the food surface.
Fisher discloses use of a microwave cooking package consisting of a single layer of polyaniline coated onto a "Dacron" polyester cloth placed on top of a "Teflon" polytetrafluoroethylene plate. See Example 2. However, this approach is not entirely satisfactory in that it is limited to microwave applications which employ porous susceptor materials. Further, processing difficulties are inherent in the use of the Fisher coating technique in that no mechanism is disclosed for obtaining uniform or controlled polymer deposition on the substrate. Thus, absent is a mechanism or suggestion of how to control heat profile characteristics in the susceptor material.
Further, attempts in the art to utilize conductive polymers in microwave and other applications has presented problems because of their instability in air, insolubility in both organic and inorganic solvents making them difficult to process, and variable conductivity.
The prior art has sought to enhance the stability of conductive polymers by incorporating atoms such as sulfur, nitrogen, and oxygen into the polymer backbone. Polypyrrole, a chain of five-membered rings, each of which contains a nitrogen atom, remains stable in the atmosphere indefinitely. Analogous compounds with sulfur and oxygen atoms in the nitrogen position are polythiophene and polyfuran which are stable and conductive.
Processing resistance of conductive polymers, in part, is a consequence of the fact that they form rigid, tightly packed chains which are essential if electrical charges are to jump from one molecule to the next as current moves through the polymer. The tight packing also prevents the polymer chains from intermixing with solvent molecules, making the polymer as a whole a hard, insoluble mass and thus unprocessible.
To improve the processibility of conductive polymers the art has proposed that solubility be enhanced by incorporating aliphatic side chains to the monomer molecule of the conductive polymer. Processibility may also be enhanced by incorporating a chemical oxidant in a host polymer film and then exposing the film to a monomer vapor resulting in a conductive polymer inside a processible film matrix. Additionally, electrochemically produced conductive polymer films are thin and brittle making them difficult to process.
U.S. Pat. Nos. 4,604,427 to Roberts and 4,521,450 to Bjorklund disclose methods for producing stable conductive polymers. Roberts introduces a conductive polymer onto the surface layer of a host polymer and polymerization thereof is initiated through exposure to an oxidant initiator. Bjorklund describes a method of depositing a coating of a doped conductive polypyrrole on a solid impregnable base. However, both methods are multistep processes with an economic disadvantage for bulk production.
U.S Pat. No. 4,803,096 to Kuhn et al. discloses a method for creating electrically conductive fabrics. Reaction conditions are controlled so that there is epitaxial deposition of the monomer of the conductive polymer onto the textile fiber and thereafter polymerization occurs. Kuhn teaches use of doping procedures for enhancing electrical activity in conductive polymers.
There is a need in the art for a nonporous, flexible microwave susceptor material including conductive polymers and fibrous material which may be engineered to meet specific heat profile requirements for microwave cooking applications. To this end processes are required to enhance the solubility and conductivity of polymers. This invention is directed to provision of such processes and paper-like flexible, non-porous products which have diverse packaging applications in the microwave food industry. It will be appreciated that advantage would be obtained by providing such an alternative to metallic conductor or semiconductor films as microwave susceptor materials.
Accordingly, it is a broad object of the invention to provide electrically conductive polymeric materials which utilize polymers having microwave susceptive characteristics.
A more specific object of the invention is to provide a method which enhances the processibility of conductive polymers while maintaining high microwave interactivity.
Another object of the invention is to provide a method for producing electrically conductive polymeric materials which are less complex and improved over the prior art.
A further object of the invention is to provide low cost, flexible food packaging which can be used in microwave cooking that incorporates electrically conductive polymeric materials.
A further specific object of the invention is to provide microwave paper-like food packaging in which the basis weight and relative amounts of electrically conductive polymeric materials may be varied to accommodate specific heat profile requirements of food products.